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Question about SPI

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Posted by Nick Carter

on Oct 22 2011 23:21 PM

Hi there,

I'm gonna use SPI module in MSP430F5418A to accomplish full duplex SPI communication with CPLD. Do I have to write the TX function and RX function together?

And there is a register named modulation control in SPI module. What does it mean? And how to use it? Do I have to set the value of this register when I use SPI

module?

Thanks

Nick

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Posted by Arte8bits When technology becomes art

on Oct 25 2011 08:55 AM

Verified Answer

Verified by Nick Carter

Prodigy230 points

Hi Nick,

You should write one function for TX and a second function for RX. If you're a master, please note that for RX you should send NULL bytes (0x00 or 0xFF depending your configuration) in order to put the clock signal for getting the slave data.

MSP430 UART has 2 registers for its configuration: divider and modulation, combining these two registers you will set your baud rate. This is done following the next equation:

http://www.javierlongares.com/arte-en-8-bits/wp-content/uploads/2011/10/baud-rate-MSP430.jpg

But don't worry about the math, TI proposes in its datasheets a complete combination of values for the typical baud rates. So, use it!

I've written a MSP430 guide in spanish and may be it will be usefull for you: http://www.javierlongares.com/arte-en-8-bits/msp430-una-guia-de-referencia-en-castellano/

Best regards,

Javier

---

www.javierlongares.com

http://www.javierlongares.com/arte-en-8-bits

Posted by Jens-Michael Gross

on Oct 25 2011 14:11 PM

Guru141810 points

Arte8bits When technology becomes art
MSP430 UART has 2 registers for its configuration: divider and modulation

In synchronous mode (SPI or I2C), the master provides the clock, so there is no need to adjust the clock to a specific frequency. A few Hz (or kHz/MHz) more or less are unimportant as long as teh minimum and maximum specs of the slave are not violated.
So the modulation is disabled in SPI or I2C mode. It is only required in UART mode, where both peers need to work on the same frequency but do not share a clock signal.

So if your clock (e.g. SMCLK) is 8MHz and you want a frequency above 1MHz and below 5MHz, you can use a divider of 8, 4 or 2 for 1, 2 or 4MHz SPI clock. But from a clock of 8MHz, you cannot get 5MHz SPI clock. Only integer fractions of the base frequency. But as I said, it is rather unimportant, since the master generates one clock signal for each bit on teh transfer. So the clock can even change in the middle of a byte transfer without any negative effect.

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